In recent years, wavelength division multiplexing (WDM) networks using a WDM method have been established as optical communication networks. The WDM networks include optical transmission apparatuses as nodes for adding or dropping channels to/from the wavelength-multiplexed channels.
In particular, many WDM networks that serve as local (metro) networks often have a network structure including many optical transmission apparatuses arranged in a ring networks or a mesh networks. The number of optical transmission apparatuses included in a WDM network is predicted to increase with the expansion of the WDM network.
Each optical transmission apparatus includes various types of optical components, which are connected to each other via optical transmission lines, such as optical fibers. Therefore, as the number of optical transmission apparatuses in the WDM network increases, the number of optical transmission lines via which the optical components in the optical transmission apparatuses are connected to each other greatly increases. The increase in the number of optical transmission lines makes the continuity test of the optical transmission lines for the maintenance and operation of the apparatuses complex.
Accordingly, various techniques for increasing the efficiency of the continuity test of the optical transmission lines in the apparatuses have been considered. For example, a technique is known in which amplified spontaneous emission (ASE) light emitted from an amplifier is used. According to this technique, an optical transmission apparatus outputs the ASE light from the amplifier to all of the optical transmission lines included in the apparatus, and detects optical power of the ASE light at each optical transmission line by a photodiode (PD) provided at the optical transmission line. Thus, the optical continuity is checked for all of the optical transmission lines in the apparatus.
According to the above-described technique, in the optical continuity test of the optical transmission lines, excitation light is supplied to the amplifier and the ASE light emitted from the amplifier is amplified to a level such that the ASE light may be detected by the PDs. Therefore, if WDM signal light is directly input to the amplifier, the WDM signal light is amplified in addition to the ASE light by the excitation light supplied to the amplifier, which reduces the communication reliability. This may be avoided by temporarily stopping the input of the WDM signal light. However, if the input of the WDM signal light is temporarily stopped, the communication stability is reduced.